Rainforests May Be Rain Makers, Says Overlooked Theory

NEW YORK (May 1, 2009)—Climatologists have long assumed that vast rainforests are largely a regional consequence of heavy rain fall, but these ecosystems may actually be huge water pumps that generate most of their rain, says a Wildlife Conservation Society researcher in the most recent edition of BioScience Magazine.

In fact, climate experts have largely ignored a recently published hypothesis on how rainforests influence regional hydrology—called the “biotic pump”—that could explain why regions such as the Amazon remain so wet, said WCS researcher Douglas Sheil, a co-author of the BioScience Paper and Director of the Institute of Tropical Forest Conservation.

Sheil’s article titled “How Forests Attract Rain: An Examination of a New Hypothesis” explains how an overlooked study by Russian researchers Anastassia Makarieva and Victor Gorshkov could become a game-changer in terms of explaining how large rainforests such as the Amazon and Congo Forest Basin could drive the global water cycle.

“If proven, this theory could explain why continental interiors with huge rain forests remain so moist, which will lead to more accurate predictive models of climate,” said Sheil. “It could also underline the dangers of widespread deforestation in the world’s major rainforests.”

Specifically, the new predictive model explains the physics of how forested regions may pull in enormous quantities of water vapor from adjacent regions and water bodies, with an emphasis on the role of evaporation and condensation in producing differences in atmospheric pressure. As water vapor condenses into rainfall, the process creates a drop in atmospheric pressure that draws in more water vapor from surrounding areas. More water condenses than evaporates, creating a positive feedback loop.

The hypothesis explains how in coastal areas without forests, water vapor is drawn out toward the ocean where condensation levels are much higher and atmospheric pressures lower. As a result, rainfall in non-forested coastlines drops off exponentially inland with distance, as illustrated in regions such as Patagonia, Australia, and West Africa.

By contrast, forested coasts draw in vapor from the ocean due to a positive feedback process that leads to higher levels of condensation, and the largest swaths of rain forest (as illustrated by the Amazon rain forest) transport enormous quantities of water inland, hence the term “biotic pump”; it is in these locations where rain levels in continental interiors are as high as the coastal areas. The starkest prediction made by Makarieva and Gorshkov is the decrease of more than 95 percent rainfall in interior regions with significant deforestation, as opposed the 20 to 30 percent drop predicted by conventional climate models.

“The biotic pump may be the missing element needed to formulate more accurate climate models that will guide both conservation efforts and perhaps reforesting regions where desertification is a threat,” said Sheil, adding that the process of deforestation in coastal areas could dry out continental interiors.

While the theory holds promise in explaining regional and global rain fall patterns, Sheil points out that additional data from several fields are needed to further support the biotic pump theory, including: more data on how variations in landform and vegetation cover types influence air circulation patterns; the role of fire damage in forest degradation; the role of different vegetation types (i.e. coniferous and deciduous forests) in climate ; and an examination of past climates, especially instances where human activities have altered regional rainfall and climate.

Sheil points out that current theory cannot explain why wet continental interiors stay so wet, whereas the “biotic pump” theory could be the answer.

WCS works to conserve rainforests in order to mitigate the effects of climate change. In 2008, WCS and the Government of Madagascar announced a landmark agreement, where the government will sell more than nine million tons of carbon offsets to help safeguard this African nation’s most pristine forest. WCS has also initiated a predictive climate model program to accurately predict where sea ice—a vital habitat for polar bears and seals that they prey on—will remain into the near future.

The Wildlife Conservation Society saves wildlife and wild places worldwide. We do so through science, global conservation, education and the management of the world's largest system of urban wildlife parks, led by the flagship Bronx Zoo. Together these activities change attitudes towards nature and help people imagine wildlife and humans living in harmony. WCS is committed to this mission because it is essential to the integrity of life on Earth. Visit: www.wcs.org

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